Desiccant air dryers differ from cooling-based dehumidifiers in that desiccants attract moisture from air by creating an area of low vapour pressure at the surface of the desiccant, instead of cooling the air to condense moisture. When the desiccant is cool and dry, its surface vapour pressure is low and it can attract moisture from ambient air, which, when humid, has a high vapour pressure. When the desiccant attracts moisture from the air, the desiccant becomes wetter, and rises in temperature due to the release of heat from the moisture of the air being dried. At some point, the desiccant will become sufficiently wet and hot that its surface vapour pressure is no longer significantly higher than the vapour pressure of the ambient air. At that point, the desiccant will no longer attract moisture from the air and will need to be dried or reactivated before being used to dry air again. In order to dry the desiccant, heated reactivation air can be blown through the desiccant, thereby heating the desiccant so that its vapour pressure is now higher than that of the heated reactivation air, and moisture is removed from the desiccant into the heated reactivation air. When the desiccant has been sufficiently dried, it is cooled, and can dry air again.
The foregoing description suggests a limitation of air dryers in circumstances where continuous dehumidification is required; namely, the moisture absorbing capability of the desiccant is periodically exhausted and must be restored before the desiccant can be used to dehumidify air again. Various solutions to this problem have been devised involving (1) moving desiccants between a reactivation air flow and a process air flow such that the process air is always being dehumidified by part of the desiccant while the dehumidifying capacity of the rest of the desiccant is being restored by the reactivation air flow; or, (2) having at least two separate air dryers, and periodically switching the reactivation air flow and the process air flow between these two separate air dryers such that process air is always being dehumidified by one of the desiccants while the other desiccant is being dried by the reactivation air.
One known desiccant arrangement that falls within the second of the above-described categories is a two-tower arrangement in which the two separate air dryers take the form of two separate packed towers. One end of each tower can switch between receiving process air via a process air inlet, and discharging reactivation air via a reactivation air outlet. The opposite end of each tower can switch between receiving reactivation air via a reactivation air inlet, and discharging process air via a process air outlet. This solid packed tower desiccant arrangement includes two valves that are operated together. A first valve controls the process air inflow and the reactivation air outflow, while the second valve controls the process air inflow and the reactivation air outflow. The valves are operated together such that when the first valve directs process air into a first tower and receives reactivation air from the second tower, the second valve is set to receive process air from the first tower and to direct reactivation air to the second tower. The setting of both valves are changed simultaneously to switch the reactivation air flow and the process air flow between the towers.
The tower arrangement suffers from the disadvantages that (1) it requires lots of room and equipment including complicated control mechanisms to redirect the reactivation air and process air periodically, and (2) it requires a high-pressure air supply. These problems render the tower arrangement inapplicable for lower-capacity and lower-pressure usage in small areas, such as in a small boat or in recreational vehicular homes.